1. Field of the Invention
The present invention relates to a sound or audio output device, and method capable of preventing the generation of pop noise or the like in an audio system or the like.
2. Description of the Related Art
In an audio system for example, noise called “Putt”, or so-called pop noise, is generated from headphones, earphones or a speaker at power-on and power-off or the like. Techniques for reducing such pop noise have been proposed in, for example, Japanese Unexamined Patent Publication No. Hei 7(1995)-264066 and Japanese Unexamined Patent Publication No. 2005-159871 or the like.
A technique of using a digital/analog (hereinafter called “D/A”) conversion apparatus capable of preventing the generation of pop noise and the like at power-on and power-off has been described in Japanese Unexamined Patent Publication No. Hei 7 (1995)-264066. This technique relates to the D/A conversion apparatus having a D/A converter which converts a digital signal to an analog signal, a left channel amplifier (hereinafter called “amp”) and a right channel amp both of which amplify the output signal of the D/A converter, and two low-pass filters which remove high-frequency noise from output signals of the two amps. The D/A conversion apparatus is provided with two logic buffers with chip enable terminals, which have the output signal of the D/A converter input thereto and which send an output signal to the two amps; and a control circuit which sends a disable signal to the chip enable terminals during a source unstable period and which sends an enable signal to the chip enable terminals in a state in which a power source or power is stable.
A technique of using a pop noise reduction circuit capable of reducing pop noise generated at power-on and power-off of an audio amplifier has been described in Japanese Unexamined Patent Publication No. 2005-159871. The pop noise reduction circuit is provided with an A/D converter which supplies output data obtained by analog/digital (hereinafter called “A/D”)-converting an output signal of an amplifier for amplifying an audio signal (sound signal); a control apparatus which fetches the output data, and which supplies to the amplifier data for giving a bias voltage for causing the output signal of the amplifier to assume a signal having a frequency of an audible band or a signal less unreproducable by a speaker, according to the output data; and a D/A converter which receives the supplied data, D/A-converts the same and supplies it to the amplifier as a bias voltage.
FIG. 11 is a schematic configuration diagram showing a conventional general audio output device. The audio output device has a D/A converter 1 which converts a digital audio signal to an analog audio signal. An amp 3 is connected to an output of amp 3 via a coupling capacitor 2. A left channel amp 4 and a right channel amp 5 are connected to the output of the amp 3. A speaker 6 having a load resistance of 8 μl is connected to the output of the two amps 4 and 5. In this type of audio output device as shown in FIG. 11, when a digital audio signal is input, the digital audio signal is converted to an analog audio signal by the D/A converter 1. The analog audio signal is amplified by the amp 3 via the coupling capacitor 2. Further, a signal output from the amp 3 is amplified by the left channel amp 4 and the right channel amp 5 respectively, and the amplified signal is thereafter output as an audio signal from the speaker 6.
In the circuit shown in FIG. 11, when the D/A converter 1 and the amps 3 through 5 are different in source voltage and the audio signals differ in center voltage from each other, coupling capacitor 2 performs alternating current (hereinafter called “AC”) coupling to thereby separate direct current (hereinafter called “DC”) components. Further, at power-up (power-on) and power-down (power-off), coupling capacitor 2 assumes the role of preventing addition of a DC offset voltage of the D/A converter 1 to DC offset voltages of the amps 3 through 5, and preventing the output of pop noise.
In general, an audible sound determinable to be pop noise corresponds to a change of 10 mV or more in DC level, and is defined as a frequency component up to 20 kHz. As described in Japanese Unexamined Patent Publication No. 2005-159871, there is also known a method of raising a reference voltage of each amp stepwise by a D/A converter and thereby reducing pop noise.
However, when the method for inserting the coupling capacitor 2 between the D/A converter 1 and the amps 3 through 5 is implemented by a semiconductor device, the conventional audio output device needs to operate at the center voltages (reference voltages) on the amps 3 to 5 sides, in order to prevent the generation of pop noise or the like. At this time, a problem arises in that since the input side of the amp 3 assumes a high-pass filter (high-frequency band-pass filter) having coupling capacitor 2 and resistance when the amp 3 is configured by for example an inverting amplifier having an input resistor, a capacitance value of 1 μF or so is required considering a pass area of an audio signal, thereby necessitating a huge layout area. When the sheet capacitance value of the coupling capacitor 2 is 1 fF [μm2], for example, a layout area of 31.6 [mm]×31.6 [mm] is required. In order to be implemented realistically, the semiconductor device would need an external capacitance, with the resultant drawback of additional of external parts.
As described in Japanese Unexamined Patent Publication No. 2005-159871, the method of controlling the reference voltages of the amps 3 through 5 is accompanied by problems that a control circuit is additionally required, and the layout area increases.